This invention relates generally to particle study devices having multiple apertures through which particles suspended in a fluid are passed and more particularly is concerned with balancing the individual signals produced by equal sized particles passing through each of the apertures to eliminate the variations in the signals caused by the physical differences between apertures.
A particular device for studying particles of microscopic size suspended in a fluid of electrolyte whose electrical impedance or resistivity is substantially different from that of the particles is shown and described in U.S. Pat. No. 3,259,842. The fluid is passed through a microscopic aperture formed in an insulating wall. Simultaneously, an electrical current is established in the aperture providing a sensing zone whose impedance is changed in proportion to the size of the particle passed through the zone. The change in impedance is detected and a signal is generated whose amplitude is proportional to the particle size and whose duration is equal to the time that it required for the particle to pass through the sensing zone.
The signals can be counted for any given volume of suspension passed through the zone to determine the particle concentration or the signals can be segregated according to size and number to determine the particle size distribution of the suspension. The particle information derived from the signals is utilized in hospitals and laboratories and it is extremely critical that the information be accurate. The devices are typically utilized in large complex apparatus whose operators do not have the time and/or may not have the expertise constantly to monitor and correct every phase of operation of the apparatus.
Various types of apparatus have been developed to overcome such problems as an aperture being blocked by particles or other problems which cause erroneous information to be developed. One device developed to overcome the problems presented in employing a single aperture device is shown in U.S. Pat. No. 3,444,463 which is incorporated herein by reference.
In this system the sample fluid is passed through three apertures simultaneously and separate respective detecting signals are developed in response to the particles passing through each of the three apertures. The signals developed by each aperture are compared and then by a process commonly known as "voting", should one of the signals developed be beyond a predetermined limit from the average of the other two signals, that aperture is considered to be malfunctioning and the signal from that aperture is ignored in the processing of information using such signals.
The probability of more than one aperture becoming blocked or otherwise malfunctioning at the same time is remote. The reliability of the information received from the particle study is thus enhanced as a blocked or otherwise faulty aperture is ignored in the study.
A system utilizing the multiple apertures and voting as described above is disclosed in U.S. Pat. No. 3,549,994 which is also incorporated herein by reference. This system was developed especially for use in the field of medicine and biology for studying body fluids. As is well known body fluids such as blood are studied to obtain information to be used in the diagnosis and treatment of patients. The need for accuracy of this information is thus very critical.
Blood is composed of microscopic cells or particles suspended in a serum and various of these cells are important in the study of the blood. Three types of blood cells may be of particular interest including red and white blood cells which are on the order of seven or more microns in size and platelets which may range from one to four microns in size.
There are several important parameters involving these cells which are utilized in the diagnosis and study of the blood. Three of these parameters are the red blood cell count (RBC), the white blood cell count (WBC) and the mean corpuscular volume (MCV). These parameters are directly measured in the system described in U.S. Pat. No. 3,549,994.
The features of these systems are incorporated in a commercial system sold throughout the world as a COULTER COUNTER.RTM. particle analyzing instrument (the mark COULTER COUNTER.RTM. is the Registered Trademark, Registration Number 679,591 of Coulter Electronics, Inc. of Hialeah, Florida).
In the systems utilizing multiple apertures and voting, the need for accuracy of the detecting signals developed by the passage of the particles through each aperture is extremely important. If the signals which are "voted" on by the voting apparatus are not equal for the same sized particles for any reason when the circuitry may vote an aperture which is functioning normally. Even if the signal differences are not sufficient for one aperture to be voted out the resultant data and information developed in the particular study will not be accurate.
A determination of the RBC, WBC and MCV has required eight adjustments heretofore to avoid having a normally functioning aperture voted out and to develop accurate data. The three amplifiers, commonly called "pre-amps", for the white cell and the three amplifiers for the red cell study were individually adjusted at some low gain point by attenuation to balance the system.
Two of the output signals from the three red cell amplifiers are utilized to determine MCV. Two more attenuating adjustments were made, heretofore, to balance the MCV output. This made a total of eight adjustments, each of them attenuating the output signals and changing the signal to noise ratio of the electronic circuits.
The severity of the requirements of balancing the systems is greater when smaller cells such as the platelets of one to four micron size are required to be detected. The apertures in the above systems typically have had a diameter on the order of 100 to 150 microns. To obtain a better volume distribution and to increase the sensitivity of the devices to detect the smaller particles apertures of 50 microns in diameter have been used and are now being contemplated for universal use in these devices.
The apertures are formed in wafers of sapphire or ruby and have typical manufacturing tolerances of approximately one micron. This variation in the aperture size becomes critical because, using the circuitry in U.S. Pat. No. 3,259,842, the amplitude of the signal developed in response to a given size particle passing through an aperture is inversely related to the square of the diameter of the aperture. The peak signal amplitude is directly related to particle volume.
When apertures are utilized in parallel in multiple aperture systems the variance in signals caused by different aperture sizes must be corrected. As noted above the actual electronic gain of each following circuit (such as the amplifiers) might be adjusted; however, this also changes the output noise component of the signals generated. Ideally, the signal to noise ratio should be optimized and henceforth left unaltered. Utilizing 100 micron apertures with a one micron tolerance in diameter and length there exists on the order of a three percent error in the gain between the apertures when they are used in parallel.
In utilizing fifty micron apertures, however, the one micron manufacturing tolerance is equivalent to a possible six percent gain error between the three apertures used in parallel.
It would thus be desirable to balance the signals developed by equal sized particles passing through the parallel apertures without decreasing the sensitivity of the circuits which produce the necessary and critical information from the detected signals.